The front of the United Nations headquarters in the Iraqi capital Baghdad collapsed today as a massive explosion tore through the building.

At least three people were killed and scores were injured, according to reports. Residents as far as a mile away said their windows were blown out and black smoke rose hundreds of feet into the air.

The US army said the blast was caused by a car bomb, and the UN said its special representative to Iraq, Sergio Vieira de Mello, was among those hurt.

Television pictures showed rescuers scrambling over the wreckage of the building, where several floors appeared to have collapsed on top of each other.

US troops shouted as they cleared the surrounding streets of residents.

The UN confirmed its special representative to Iraq, Sergio Vieira de Mello, was among the injured.

UN worker Salim Lome told BBC News 24 that Mr de Mello was still trapped inside the wreckage of the building.

"People can see him and they can pour water over him, but he is still in the rubble," he said.

Mr Lome said the blast was directly under the office of the UN chief, which was wrecked.

"It was a huge bomb," said Mr Lome, who escaped with minor injuries.

Despite the reports, Mr Lome, told UN headquarters in New York that a number of people were hurt in the explosion, but no one was killed, as far as he knew.

He said the explosion destroyed a significant part of the building, UN spokesman Fred Eckhard said in New York.

The UN headquarters was based in a former hotel - the Canal - and its weapons inspectors worked out of the building before the war.

An Associated Press reporter at the scene saw nine people being carried out of the hotel on stretchers.

Nazar Hababa, a UN driver, was covered in blood as he recounted seeing Iraqis and foreigners injured in the rubble.

There were conflicting reports about what caused the blast, and at least one witness said they thought it had been caused by a rocket.

"At 4:30pm we heard a big explosion. It was caused by a rocket, said Adnan Al-Jabouri, a second UN driver at the hotel.

Dozens of American Humvees were at the scene and Black Hawk helicopters hovered above. Some were ferrying the wounded away for treatment.

Meanwhil;e it was reported that a soldier from the US Army's 1st Armoured Division was killed by an explosive device in Baghdad yesterday as the top US civilian administrator said that attacks by saboteurs on Iraq's decrepit infrastructure and oil industry have cost the economy billions of dollars.

* Former Iraqi vice president Taha Yassin Ramadan, 20th on the US Most Wanted list, was captured in the north of the country and handed to US forces in Mosul today, the Pentagon said. He was said to be disguised as a peasant.